1. Field of the Invention
This invention relates to a method for forming an energy subtraction image wherein, from a plurality of radiation images, an energy subtraction image is formed which includes little noise and which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .sym.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order for the desired image density to be obtained, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal (image signal) to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT.
In the radiation image recording and reproducing systems wherein recording media, such as X- ray film or stimulable phosphor sheets, are used, subtraction processing techniques for radiation images are often carried out on image signals detected from a plurality of radiation images of an object which have been recorded on the recording media.
With the subtraction processing techniques for radiation images, an image is obtained which corresponds to a difference between a plurality of radiation images of an object recorded under different conditions. Specifically, a plurality of the radiation images recorded under different conditions are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals which represent the image information recorded at corresponding sampling points in the radiation images are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order for the image of a specific structure (for example, a blood vessel) of an object to be extracted from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure (for example, a blood vessel) of the object is enhanced by the injection of contrast media. In the latter method, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation with different energy levels. Specifically, an object is exposed to several kinds of radiation with different energy levels, and a plurality of radiation images are thereby obtained in which different images of a specific structure are embedded. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure. The applicant proposed novel energy subtraction processing methods using stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037.
A plurality of radiation images, which are subjected to energy subtraction processing, will herein be referred to as the "original images". An image signal representing a subtraction image is obtained by subtracting the image signals representing the original images from each other. Therefore, the image signal representing the subtraction image has a lower signal-to-noise ratio (S/N ratio) than the image signals representing the original images. As a result, the problems occur in that the image quality of the subtraction image becomes worse than the image quality of the original images.
By way of example, energy subtraction processing is often carried out in the manner described below. Specifically, an object, such as the chest of a human body, which is constituted of soft tissues and bones, is exposed to several kinds of radiation with different energy levels, and a plurality of radiation images of the object are thereby obtained. The plurality of the radiation images are read out, and a plurality of image signals representing the radiation images are generated. Energy subtraction processing is then carried out on the plurality of the image signals. From the energy subtraction processing, a soft tissue image signal is obtained which represents a soft tissue image primarily composed of patterns of the soft tissues of the object. Alternatively, a bone image signal is obtained which represents a bone image primarily composed of patterns of the bones of the object. Thereafter, the soft tissue image is reproduced as a visible image from the soft tissue image signal, or the bone image is reproduced as a visible image from the bone image signal. In the soft tissue image, the patterns of the bones have been erased. Therefore, patterns, which were behind the bone patterns or were rendered imperceptible by the bone patterns in the original images, become more perceptible in the soft tissue image than in the original images. Also, in the bone image, the patterns of the soft tissues have been erased. Therefore, patterns, which were behind the soft tissue patterns or were rendered imperceptible by the soft tissue patterns in the original images, become more perceptible in the bone image than in the original images. Accordingly, a subtraction image can be obtained which is well matched to the purposes of diagnosis. However, because the soft tissue image and the bone image re obtained from the subtraction processing, the problems occur in that noise components have been emphasized in the soft tissue image and the bone image than in the original images. From this point of view, the image quality of the soft tissue image and the bone image could not heretofore been kept good.
Accordingly, in U.S. patent application Ser. No. 654,450, the applicant proposed a first method for forming an energy subtraction image wherein, from a plurality of original images, a subtraction image is formed in which noise has been reduced to approximately the same level as that in original images and which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The proposed first method comprises the steps of:
i) after a plurality of radiation images (e.g., a radiation image recorded with radiation having a high energy level and a radiation image recorded with radiation having a low energy level) of an object are recorded on recording media by irradiating several kinds of radiation with different energy levels (e.g., radiation having a high energy level and radiation having a low energy level) to said object, which is constituted of a plurality of tissues (e.g., bones and soft tissues) exhibiting different levels of radiation absorptivity with respect to the several kinds of radiation with different energy levels, and a plurality of original image signals representing the plurality of said radiation images (e.g., an image signal representing the radiation image recorded with the radiation having a high energy level and an image signal representing the radiation image recorded with the radiation having a low energy level) are then detected,
generating a first image signal (e.g., a bone image signal), which represents a first image primarily composed of patterns of first tissues (e.g., the bones) of said object, from the plurality of said original image signals,
ii) generating a first smoothed image signal by processing said first image signal (e.g., the bone image signal), said first smoothed image signal representing a first smoothed image in which noise components of said first image have been reduced or eliminated, and
iii) generating a second image signal (e.g., a soft tissue image signal) by subtracting said first smoothed image signal from an original image signal, said second image signal representing a second image primarily composed of patterns of second tissues (e.g., the soft tissues) of said object.
The proposed first method for forming an energy subtraction image may be embodied in various, substantially identical manners. For example, each of the steps of the first method for forming an energy subtraction image may be divided even further into a plurality of steps. Alternatively, the operations may be carried out in different orders.
By way of example, as one of the embodiments which are substantially identical with the aforesaid first method for forming an energy subtraction image, an embodiment has been proposed which comprises the steps of:
i) after a plurality of radiation images (e.g., a radiation image recorded with radiation having a high energy level and a radiation image recorded with radiation having a low energy level) of an object are recorded on recording media by irradiating several kinds of radiation with different energy levels (e.g., radiation having a high energy level and radiation having a low energy level) to said object, which is constituted of a plurality of tissues (e.g., bones and soft tissues) exhibiting different levels of radiation absorptivity with respect to the several kinds of radiation with different energy levels, and a plurality of original image signals representing the plurality of said radiation images (e.g., an image signal representing the radiation image recorded with the radiation having a high energy level and an image signal representing the radiation image recorded with the radiation having a low energy level) are then detected,
generating a first image signal (e.g., a bone image signal), which represents a first image primarily composed of patterns of first tissues (e.g., the bones) of said object, and a second image signal (e.g., a soft tissue image signal), which represents a second image primarily composed of patterns of second tissues (e.g., the soft tissues) of said object, from the plurality of said original image signals,
ii) generating a noise image signal by processing said first image signal (e.g., the bone image signal), said noise image signal representing a noise image in which components representing the patterns of said first tissues and primarily constituting low spatial frequency components of said first image have been reduced or eliminated, and
iii) generating a new second image signal by adding said noise image signal to said second image signal (e.g., the soft tissue image signal), said new second image signal representing a new second image primarily composed of the patterns of said second tissues (e.g., the soft tissues) of said object.
The applicant also proposed a second method for forming an energy subtraction image, which comprises the steps of:
i) after a plurality of radiation images of an object are recorded on recording media by irradiating several kinds of radiation with different energy levels to said object, which is constituted of a plurality of tissues exhibiting different levels of radiation absorptivity with respect to the several kinds of radiation with different energy levels, and a plurality of original image signals representing the plurality of said radiation images are then detected,
carrying out a first process for generating a first image signal, which represents a first image primarily composed of patterns of first tissues of said object, from the plurality of said original image signals,
thereafter carrying out a second process, which comprises the steps of:
a) generating a first smoothed image signal by processing said first image signal, said first smoothed image signal representing a first smoothed image in which noise components of said first image have been reduced, and PA1 b) generating a second image signal by subtracting said first smoothed image signal from an original image signal, said second image signal representing a second image primarily composed of patterns of second tissues of said object, and PA1 a) generating a second smoothed image signal by processing said second image signal, said second smoothed image signal representing a second smoothed image in which noise components of said second image have been reduced, and PA1 b) generating a new first image signal by subtracting said second smoothed image signal from an original image signal, said new first image signal representing a new first image primarily composed of the patterns of said first tissues of said object.
iii) thereafter carrying out a third process, which comprises the steps of:
An image having better image quality can be obtained by repeating the second and third processes in the aforesaid second method for forming an energy subtraction image. Specifically, the applicant further proposed a third method for forming an energy subtraction image, which comprises the steps of, after the processes in the aforesaid second method for forming an energy subtraction image have been carried out, repeating the following once or several times:
i) a new second process for generating a new second image signal by carrying out said second process in which said new first image signal obtained from said third process is taken as said first image signal in said second process, said new second image signal generated by said new second process representing a new second image primarily composed of the patterns of said second tissues of said object, and
ii) a new third process for generating a new first image signal by carrying out said third process in which said new second image signal is taken as said second image signal in said third process, said new first image signal generated by said new third process representing a new first image primarily composed of the patterns of said first tissues of said object.
By applying the aforesaid second or third method for forming an energy subtraction image, a new second image signal can be generated ultimately which represents a new second image primarily composed of the patterns of the second tissues of the object. Specifically, the applicant still further proposed a fourth method for forming an energy subtraction image, which comprises the steps of:
after the processes in the aforesaid second or third method for forming an energy subtraction image have been carried out,
generating a new second image signal by carrying out said second process or said new second process in which said new first image signal obtained from said third process or said new third process is taken as said first image signal in said second process or said new second process, said new second image signal thus most recently generated representing a new second image primarily composed of the patterns of said second tissues of said object.
Each of the aforesaid second to fourth methods for forming an energy subtraction image includes steps similar to those of the aforesaid first method for forming an energy subtraction image. Therefore, as described above with reference to the first method for forming an energy subtraction image, the second to fourth methods for forming an energy subtraction image embrace various, substantially identical embodiments. Also, other steps, such as noise reducing processes, may be carried out before or after the aforesaid first to fourth methods for forming an energy subtraction image.
The terms "first image" and "second image" (or the terms "new first image" and "new second image") as used herein for the aforesaid first to fourth methods for forming an energy subtraction image mean two images, which have been obtained from energy subtraction processing and in which the patterns of different tissues of a single object have been emphasized or only such patterns are illustrated. The first image and the second image (or the new first image and the new second image) are not limited to specific images. For example, the first image and the second image (or the new first image and the new second image) may be a soft tissue image and a bone image. Alternatively, in cases where the object is a mamma of a human body, the first image and the second image (or the new first image and the new second image) may be an image, in which the patterns of mammary glands have been emphasized, and an image, in which the pattern of a malignant tumor has been emphasized.
The aforesaid first to fourth methods for forming an energy subtraction image are based on the findings that, because an image signal representing a subtraction image is obtained by subtracting the image signals representing the original images from each other, the image signal representing the subtraction image has a lower S/N ratio than the image signals representing the original images.
Specifically, with the aforesaid first method for forming an energy subtraction image, the first image signal, which represents a first image primarily composed of patterns of first tissues of the object, is generated by carrying out a subtraction process on a plurality of original image signals. The first smoothed image signal is then generated by processing the first image signal. The first smoothed image signal represents a first smoothed image in which noise components of the first image have been reduced or eliminated. Thereafter, the first smoothed image signal is subtracted from an original image signal, and the second image signal is thereby generated which represents a second image primarily composed of patterns of second tissues of the object. Therefore, a second image can be obtained in which noise has been reduced to approximately the same level as that in the original images and which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
In order for the second image having good image quality to be obtained, it is necessary that, in the course of generating the first smoothed image signal representing the first smoothed image, the signal components of the first image signal representing the patterns of the first tissues of the object can be kept uneliminated, and only the noise components of the first image signal can be eliminated. However, part of the spatial frequency components corresponding to the patterns of the first tissues and part of the spatial frequency components corresponding to the noise components are identical with each other. Therefore, even if a non-linear filter is utilized which eliminates as many noise components as possible, the noise components and the signal components of the first image signal representing the patterns of the first tissues of the object cannot be completely separated from each other.
Accordingly, with the aforesaid second to fourth methods for forming an energy subtraction image, instead of aiming at completely eliminating the noise components only with a single noise reducing process, a plurality of noise reducing processes are carried out sequentially such that an image can be obtained in which noise has been reduced and which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
Specifically, with the aforesaid second method for forming an energy subtraction image, a noise reducing process is carried out on the first image signal in order to generate the first smoothed image signal representing a first smoothed image in which noise components of the first image have been reduced. The second image signal is then generated by subtracting the first smoothed image signal from an original image signal. Thereafter, a noise reducing process is carried out on the second image signal in order to generate the second smoothed image signal representing a second smoothed image in which noise components of the second image have been reduced. The new first image signal is then generated by subtracting the second smoothed image signal from an original image signal. With the two noise reducing processes, noise components can be reduced in appropriate manners. Therefore, an image can be obtained which contains less noise and which has better image quality and can serve as a more effective tool in, particularly, the efficient and accurate diagnosis of an illness, than the aforesaid first method for forming an energy subtraction image.
With the aforesaid third method for forming an energy subtraction image, the steps of the aforesaid second method for forming an energy subtraction image are carried out repeatedly such that more noise components can be reduced. The respective noise reducing processes can be allotted with appropriate modes of processing. Therefore, an image can be obtained in which noise components have been reduced even further.
With the aforesaid fourth method for forming an energy subtraction image, after the steps of the aforesaid second or third method for forming an energy subtraction image have been carried out, a noise reducing process is carried out on the new first image signal, which has been generated by the second or third method for forming an energy subtraction image. In this manner, a new first smoothed image signal is generated. Thereafter, the new first smoothed image signal is subtracted from an original image signal. Accordingly, the new second image can be obtained in which noise components have been reduced.
Each of the aforesaid methods for forming an energy subtraction image will hereinbelow be referred to as a method for forming a graininess-improved energy subtraction image. Also, the energy subtraction processing employed in each of the aforesaid methods for forming an energy subtraction image will hereinbelow be referred to as graininess improving energy subtraction processing.
When the graininess improving energy subtraction processing is carried out with each of the aforesaid methods for forming an energy subtraction image, by way of example, first and second recording media, such as sheets of X-ray film or stimulable phosphor sheets, are utilized. In such cases, in order to separate radiation having a high energy level and radiation having a low energy level from each other, a filter for filtering out radiation having a low energy level is located between the first and second recording media. Radiation is then irradiated to an object, and the radiation, which has passed through the object, impinges upon the combination of the first recording medium, the filter, and the second recording medium, from the side of the first recording medium. At this time, after the radiation has passed through the first recording medium, the radiation having a low energy level is filtered out by the filter, and only the radiation having a high energy level impinges upon the second recording medium. In many cases, a filter constituted of a metal, such as a copper plate, is utilized as the filter for filtering out radiation having a low energy level. Alternatively, a stimulable phosphor sheet may be utilized as the filter for filtering out radiation having a low energy level. Specifically, a stimulable phosphor sheet has the effects of filtering out radiation having a low energy level. Therefore, a stimulable phosphor sheet can be utilized in lieu of the filter for filtering out radiation having a low energy level.
In such cases, the stimulable phosphor sheet, which is located as the filter between the two recording media, can store image information of an object thereon. Therefore, it is considered that, when the image information stored on the stimulable phosphor sheet employed as the filter is utilized, the graininess of the energy subtraction image can be improved even further.